Method of operating high-intensity arc discharge lamp

ABSTRACT

The arc tube of a high-intensity arc discharge lamp is vertically operated and has an expanded section at or near its center in order to increase lamp efficiency by improving the convective flow pattern within the arc tube during normal lamp operation.

United States Patent Fohl 1 1 May 13, 1975 [54] METHOD OF OPERATING 2,166.951 7/1939 Germer 1. 313/220 x HIGHJNTENSITY ARC DISCHARGE LAMP 2,173,261 9/1939 M'arden 313/220 X 2,176,134 10/1939 Holst 1. 313/220 X Inventor: Timothy Fohl, Carhsle. ss 2.187.736 1/1940 Germer 313/220 x 1 Assignw GTE Sylvania Incorporated, FOREIGN PATENTS OR APPLICATIONS Danvers Mass 485,489 5/1938 United Kingdom 313/184 [22] Filed: July 19, 1973 Primary ExaminerNathan Kaufman [2H Appl' 380739 Attorney, Agent, or Firm.1ames Theodosopoulos [52] U.S. Cl. 313/220; 313/17; 313/220;

315/344; 313/227 [57] ABSTRACT [51 int. Cl. 11013 17/16 The arc tube of a high-intensity am discharge p is 5 n w f Search 313 220 17 227; 315 344 vertically operated and has an expanded section at or near its center in order to increase lamp efficiency by 5 References (m improving the convective flow pattern within the arc UNITED STATES PATENTS tube during normal lamp operation. 2,093,892 9/1937 Lemmers 313/17 X 3 Claims, 1 Drawing Figure METHOD OF OPERATING HIGH-INTENSITY ARC DISCHARGE LAMP BA- KGROUND OF THE INVENTION 1. Field of the Invention This invention relates to high-intensity discharge lamps (HID) which include the groups of lamps com monly known as mercury. metal halide. and high pressure sodium lamps. These lamp types are characterized as discharge devices which are wall stabilized and whose light producing envelopes have a bulb wall loading in excess of 3 watts per square centimeter.

2. Description of the Prior Art High-intensity discharge lamps have become commercially useful in the past or years because they are efficient producers of visible light, considerably more efficient than. say, incandescent lamps. HID lamps generally have an operating pressure of about I to ID atmospheres and an arc tube operating temperature of at least about 400C. Thus the arc discharge in such lamps is affected by convection currents within the arc tube. HID lamps generally have a uniform diameter arc tube, that is, an arc tube with constant cross section.

Some types of prior art compact source are discharge lamps have been made with non-uniform diameter are tubes. such as short are lamps and heavily loaded capillary lamps. However, in such lamps the arc discharge is not affected by convection currents within the arc tube. For example. short are lamps generally contain a spherical arc tube and have are discharges that are electrode stabilized. This means that the arc length is small compared to the arc tube diameter. that the shape of the arc discharge is independent of the shape of the arc tube and that the arc discharge is not affected by convection currents within the arc tube.

Some types of capillary arc discharge lamps have been made with a slight bulge at the hottest portion of the arc tube in order to prevent melting of the glass thereat. But in such lamps the arc discharge extends to the walls of the arc tube and is confined thereby; thus the arc discharge is constricted and is not significantly affected by convection currents within the arc tube. In addition, capillary lamps are so heavily loaded (watts/sqcm.) that. in general. they must be artificially cooled in order to prevent the arc tube from melting.

In contrast to compact source arc discharge lamps, the arc discharge in HID lamps is affected by convection currents within the arc tube during lamp operatron.

SUMMARY OF THE INVENTION I have discovered that a substantial unexpected increase in efficiency (lumens/watt) of HID lamps can be obtained by a change in the shape ofthe arc tube from the uniform diameter tube that is commonly used. In my invention the arc tube has an expanded diameter section intermediate the electrodes in order to control the flow of convective currents within the arc tube.

In lamps of the type with which this invention is concerned, there are opposing convective flows of the gaseous and vaporized material within the arc tube during normal lamp operation, When the arc tube is operated with its axis vertical, or at angles other than horizontal. the upward convective flow is essentially along the axis of the arc tube. which is also the axis of the core of the are discharge. The downward convective flow is near the walls of the arc tube. When the upward and downward flows are in close proximity, the shear between them causes radial convective flowsv The purpose of the present invention is to reduce the shear between the upward flow and the downward flow by expanding the diameter of the arc tube at a section intermediate the electrodes. This increases the distance between the upward flow and the downward flow; it also decreases the velocity of the downward convective flow. Reduction of the shear reduces radial mixing between the upward flow and the downward flow which results in a substantial increase in lamp efficiency.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE in the drawing is an elevational view of an HID lamp having an expanded section are tube in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing. one example of an HID lamp. a metal halide lamp. in accordance with this invention comprises an outer glass envelope or jacket 1. Jacket 1 is provided at its end with a sealed reentrant stem 2 through which extend relatively stiff lead-in wires 3 and 4 connected at their outer ends to the electrical contacts of the usual base 5. Disposed within jacket 1 is an expanded section arc tube 6.

Are tube 6 is supported within jacket 1 by means of metal frames 7 and 8 at each end of arc tube 6. Metal frames 7 and 8 comprise rigid wires 9 and I0, respectively. to which are fastened clamps 11 and 12 each of which supports a pressed seal end of arc tube 6. Metal frame 7 is supported by lead-in wire 4 to which it is welded. Metal frame 8 is supported at the other end by metal leaf springs I3 which press against the inner wall of envelope 1.

Electrical connection from lead-in wire 4 and metal frame 7 to the proximate main solid electrode 14 is through connective wire I5. Electrical connection from lead in wire 3 to the other main electrode 16 is through wires 17, 18 and 19. Electrical connection from lead-in wire 3 to the starter electrode 20 is through resistor 21. Bimetal switch 22 shorts the starter electrode to the adjacent main electrode 14 after lamp ignition occurs. The atmosphere within jacket I is an inert gas. such as nitrogen.

Arc tube 6 has an expanded section at about or near its center in order to substantially eliminate radial convective flow between the upward flow and the downward flow. The result of this is a significant increase in lamp efficiency.

In an example ofa I000 watt metal halide lamp in ac cordance with this invention. an arc tube was blow molded into the desired expanded section shape from an open-ended fused-quartz tube, 18 mm ID. by 21.6 mm O.D. by 6 inches long. The outside diameter of the molded tube was as follows: 0.851 inches for a distance of 1 inch from the upper end. then gradually increasing to a maximum of 1.194 inches for a distance of three eighths inch starting at a point 2 /2 inches from the upper end. then gradually decreasing to 0.85] inches for the lower seven-cighths inch. After the molded tube was formed into arc tube 6 by embedding the elec trodes in the press seals at each end and by adding an arc tube fill of mercury. sodium iodide. scandium iodide and inert gas and then scaling. the ratio of maximum arc tube diameter to minimum arc tube diameter was about l.4. The arc length (distance between main electrodes) was 9 mm and the ratio of arc length to maximum inside tube diameter was about 3.4. The maximum diameter of arc tube 6 was slightly above the center of the arc tube.

The efficiency of this lamp was 126.5 lumens per watt, which is about 26 percent greater than the efficiency of present commercially available 1000 watt metal halide lamps, which have an efficiency of about 100 lumens per watt. Also, radial convective flow between the upward and downward flows was almost completely eliminated in this lamp and the convective flow pattern could be considered a single convective cell extending almost the entire length of the arc. The flow pattern can be made visible by introducing fine carbon particles into the arc tube and by observing their motion during lamp operation, the carbon particles being heated to incandescence by the are. If desired. convective velocities can be readily measured by filming the motion of the particles.

In the determination of the optimum diameter for the expanded section of an arc tube in accordance with this invention, are tubes were tested whose maximum diameter varied from about one-fifth the arc length to about equal the arc length. It was found that if the maximum diameter is too small, radial flow is not eliminated; if the maximum diameter is too great, then the result is an unsteady or wavering are. ln general, an increase in the pressure within the arc tube would require an increase in the diameter of the expanded section as would, also, an increase in the density of filling material per unit length of arc length.

The maintenance of lamps in accordance with this invention appears to be about the same or slightly better than that of prior art lamps, that is to say, lamps having uniform diameter arc tubes. In one test, 23 l000-watt metal halide lamps were made having an expanded section are tube as per this invention. The average initial efficiency of these 23 lamps was 1 l9 lumens per watt. The average efficiency after 1000 hours operation was ll2 lumens per watt, a maintenance of 94 percent. This is about equal to the maintenance of prior art l000 watt metal halide lamps after 1000 hours operation, which had an average initial efficiency of 100 lumens per watt and a 1000 hour efficiency of 94 lamens per watt.

The efficiency of mercury vapor lamps in accordance with this invention is improved about [0 to 20 percent over that of prior art mercury vapor lamps, that is, those lamps having a uniform diameter are tube. The efficiency of the prior art lamps is about 55 to 60 lumens per watt while the efficiency of mercury vapor lamps as per this invention is about to lumens per watt. Mercury vapor lamps have an arc tube till of only mercury and an inert gas, the mercury being completely vaporized during lamp operation.

I claim:

1. In a high-intensity arc discharge lamp having a generally tubular arc tube disposed within an outer envelope, the arc tube having an electrode at each end, the improvement which comprises an expanded section in the arc tube intermediate the electrodes, the shape of the expanded section being such as to eliminate ra dial convective flow between the upward and downward convective flows within the arc tube during normal lamp operation.

2. The lamp of claim 1 wherein said expanded section is near the center of the arc tube.

3. The lamp of claim 2 wherein the ratio of maximum to minimum diameter of said are tube is about l.4. 

1. In a high-intensity arc discharge lamp having a generally tubular arc tube disposed within an outer envelope, the arc tube having an electrode at each end, the improvement which comprises an expanded section in the arc tube intermediate the electrodes, the shape of the expanded section being such as to eliminate radial convective flow between the upward and downward convective flows within the arc tube during normal lamp operation.
 2. The lamp of claim 1 wherein said expanded section is near the center of the arc tube.
 3. The lamp of claim 2 wherein the ratio of maximum to minimum diameter of said arc tube is about 1.4. 